Excessive supply voltage drop across area portions of an Integrated Circuit (“IC”) may impact the chip's performance and reliability and may cause problems with the chip timing and even lead to device functional failure. Therefore, an IC design must carefully take into account and avoid any possible voltage drop issues. Avoidance of such voltage drop issues, and the power integrity of the IC design generally, may be assured through good power network planning and synthesis and by accurately analyzing the design to detect problems. Voltage drop analysis occurs at two stages of IC design. The first analysis occurs immediately post “floor planning” and the second analysis occurs after standard logic cell placement and extraction.
The logic cells may serve as various functions on the integrated circuit such as, but not limited to, inverters, buffers, or generally, “repeaters” of input and output signals throughout the integrated circuit. The logic cells have a switching factor which determines how the logic cells will sink and source current. Where logic cells are clustered together on an integrated circuit, the cluster may have a corresponding voltage drop which may be unacceptable and may lead to problems such as signal timing degradation. An integrated circuit or portions of an integrated circuit may have input and output ports where the input and output ports may be clustered together in order to meet full-chip timing requirements or other logic interaction requirements of the integrated circuit. FIG. 1 illustrates an integrated circuit base area 100 which includes a plurality of signal input and output ports 101 arranged very closely together. These clustered signal ports 101 are isolated by logic cells wherein the logic cells may be, for example, inverters and/or buffers. These logic cells act as repeaters for signals coming into the block or going out of the block. The size of the logic cells is decided by various factors such as the logic cell driving capability and analysis at full-chip operating level to determine an optimal number of logic cells for meeting the desired timing and signal integrity requirements of the integrated circuit. Because the logic cells serve as repeaters they are placed as close as possible to the signal ports 101 to prevent any attenuation of the input and output signals. This leads to clustering of the repeaters near the signal ports.
FIG. 2 illustrates a known method of integrated circuit design. The method of design illustrated by FIG. 2 consists of four phases. In Phase 1 as shown in block 201, the initial design is started and the integrated circuit floor plan is created. Phase 2 as shown in block 203 is referred to a “logic synthesis.” In logic synthesis the textual data created in block 201 is mapped to the standard logic cell representations. Block 205 illustrates Phase 3, the “placement phase,” wherein standard logic cells are placed on the integrated circuit based on the integrated circuit size and also frequency and power requirements. In Phase 4, block 207, the “routing phase” is completed. In the routing phase the power distribution connections are routed across the integrated circuit to ensure integrity of data transmitted throughout.
For example, using the method illustrated in FIG. 2, after IC floor planning and logic synthesis as shown in blocks 201 and 202, and prior to placement of any standard logic cells in block 205, an analysis proceeds by assigning regions of the IC with the voltage drop (or IR drop) budget based on estimates and previous design experience. However, the results obtained may be totally inaccurate when the standard logic cells are placed in 205 and after the second analysis is carried out post-extraction. By this time in the process, considerable design hours would have been expended for the physical design of the chip, and any subsequent adjustments to fix any IR drop violations would impact the time to “tape-out” (i.e., commencement of the manufacturing process) of the IC. Therefore, obtaining an accurate logic cell placement that meets the desired voltage drop characteristic is essential for keeping IC Design time to a minimum.
One approach for IC design is referred to as “divide and conquer.” The IC may be divided into area portions or blocks, which are handled by multiple designers so that the design turn around time is reduced. Most of the blocks are custom designed so that the timing, area and power goals are achieved. However, due to the complex or otherwise irregular logic design required for the blocks to achieve the design constraints, commercial Computer Aided Design (CAD) tools are usually used for the physical design of the blocks. It is during the use of these commercial CAD tools that the designer does not have total control of the logic cell placement and this can impact the timing, area, power goals and signal integrity assumptions of the IC design. FIGS. 3 and 4 illustrate examples of logic cell placement that can occur using CAD tools as described. As shown in FIG. 3 an area 300 of an integrated circuit has a plurality of clustered logic cells 301 that are placed very close together with 100% logic cell utilization within the region. Likewise FIG. 4 shows an area 400 of an integrated circuit having a clustered set of logic cells 401. Such high cell utilization as shown in FIG. 3 and FIG. 4 leads to problems with the voltage drop in the area of the logic cells.
Voltage drop problems may also occur during the design process with respect to the IC power grid. The design of an IC power grid is usually standardized such that it may suit a wide variety of designs thereby reducing IC design cycle time. However, there may be cases where these standard power grids do not meet the voltage drop requirements. Changing the power grid design entails changing the CAD flow which is time consuming and requires additional design hours because the changes must be tracked and supported during the entire design cycle.
Some existing design methods perform iteration at the placement stage such as the placement stage shown in FIG. 2, block 205. The iterative methods for reducing supply voltage drop perform iterations of logic cell placement and corresponding voltage drop analysis of the placed logic cells. If a placement does not meet the desired voltage drop margin, the entire process is repeated again which is time consuming and increases the cycle time. Further such methods do not take into account cross talk between logic cells, signal integrity or the switching factor of the logic cells.
Another proposed design method estimates the power consumption of each logic cell and places logic cells with higher power consumption closer to the power supply sources. While this method takes into account total wire length and the power consumption of the logic cells to determine the logic cell placement, the method must recalculate the supply voltage drop during each placement iteration which unacceptably adds design cycle time. Further, such methods do not account for timing, area, routing resources, cross talk and logic cell switching factor. Therefore, it might happen that all logic cells having high power consumption are placed near each other in descending order which would create a local temperature hot spot. Further, the timing could become degraded if power consuming logic cells are placed away from the interacting logic.